The present invention relates to steam turbines, and more specifically relates to structures for connecting a source of steam to the inner cylinder of a steam turbine.
Cracking failures in the inlet sleeves of fossil steam turbines have been a significant problem for almost two decades. Over the last 15-20 years, the assignee of the present invention is aware of over 100 cases of cracking that have occurred in its turbines. Obviously, such cracking is detrimental to the steam turbine and results in increased downtime and maintenance, thereby increasing the costs of operating the steam turbine. It would therefore be desireable to identify the phenomena which contribute to such cracking and reduce or eliminate them.
FIG. 1 illustrates a typical fossil turbine 100, the inner cylinder 101, and the outer cylinder 102 to which the inlet sleeve 104 is connected. As shown by the arrows, steam flows through the inlet sleeve 104, into the inner cylinder 101 of the turbine 100, where it encounters a series of blades 105, which induce rotation in the output shaft 110. The operation and various constructions of the blades 105 and other portions of a steam turbine 100 are well known to those of ordinary skill. For purposes of the present invention, the details of the inlet sleeve 104, its connection to the outer cylinder 102, as well as the thermal and bending stresses generated within these components during operation are of primary concern.
As illustrated in cross-section in FIG. 2, in prior designs the inlet sleeve 10 is typically connected to a flexible skirt 108, which forms part of the outer cylinder 102, using a circumferential weld 106. In the failures mentioned above, cracks 51 usually occur near the inside trepan radius 53 at the fixed end 103 of the inlet sleeve 104. Metallurgical examination indicates that failures due to cracks 51 such as those illustrated have been caused by high cycle fatigue, induced by reverse bending stresses. Flow-excited vibration brings about these failures by creating the reverse bending stresses. When the structure vibrates at its resonant frequency, the cantilevered free end 109 oscillates with increasingly large amplitudes, producing alternating bending stresses at the fixed end 103. As shown by the crack 51 at the initiation site illustrated, when these stresses exceed the endurance limit of the sleeve material, fatigue cracking failures occur and will most likely occur in the manner illustrated.
One means of addressing the cracking problem is to increase the trepan radius 53 adjacent to the site of the cracks 51. Although this solution lowers the stress concentration factor in the region in which the cracks are typically initiated, failures have been shown to continue at an unacceptably high level even with the larger radius 53. Since the fundamental cause of the cracking problem is the unrestrained movement at the free end 109 of the sleeve 104, which permits excessive vibration, another means of addressing the problem would be to anchor, or otherwise fix, the free end 109 of the sleeve 104 to prevent this oscillation. However, the free end 109 cannot be anchored to the steam turbine structure since a sliding joint within the inner cylinder 101 is necessary. The free end 109 cannot be attached to the outer cylinder 102, because unacceptable thermal stresses would result from the temperature differential. Another possible solution to this problem would be to reduce the cold clearance "c" between the inlet sleeve 104 and the inner surface of the outer cylinder 102 to restrict movement. This is not a viable solution, however, since it would not allow for sufficient thermal expansion during operation, and could fracture the sleeve 104. On the other hand, too large a clearance "c" would render the structure totally ineffective for restraining the vibration.
Thus, the unique geometric, vibrational and heat transfer characteristics of the steam inlet/outer cylinder junction have posed a problem which has defied solution by conventional engineering modifications. As pointed out above, the importance of eliminating the vibration and resulting stresses at this joint cannot be overstated. It would be desirable, however, to provide a solution to the problem that does not add an undue degree of complexity to the turbine assembly.